1. Field of the Invention
The present invention concerns a method and a monitoring device for welding by means of a positioning device, in particular a welding robot.
2. Description of the Prior Art
In resistance spot welding (RSW), plates that are to be joined, for example, are pressed together by two welding electrodes and a welding current is conducted through the electrodes and the plates, wherein due to the increased transfer resistance between the plates their temperature is increased so much that the plates melt there.
If this is executed automatically by a positioning device (for example a welding robot), this device takes up predetermined poses in a position-regulated manner in order to position electrode holder and work piece(s) relative to one another. For this the positioning device can move the work piece and/or the electrode holder. For example, a welding robot can apply a robot-guided electrode holder to a stationary work piece or, conversely, can supply a gripped work piece to a stationary electrode holder.
In the approach and/or in the welding pose, the electrode holder is closed and a welding spot is generated via movement of at least one electrode towards the other electrode. The poses to set the welding spots can, for example, be “learned” in advance with manual operation or can be programmed offline and, for example, be occupied with the use of proportional-integral-differential single joint regulators.
If in operation the position (i.e. bearing and/or orientation) of a work piece to be welded relative to a tool reference system of the positioning device—for example the TCP (“tool center point”)—of a welding robot now deviates from the position relative to which the pose was predetermined—for example since plates are deformed or imprecisely mounted in a feeder tool or feeder tool and positioning device are positioned imprecisely relative to one another—the position regulation attempts to forcibly reach the reference position. In particular given high strength and super high strength plates, this can thereby lead to a degradation or, respectively, a failure of the welding process, a damage to work piece, tool and/or positioning device and the like.
Therefore, in practice electrode holders borne in a floating manner have previously been provided that decouple the electrode holder from transversal forces during the welding process. In particular given non-stationary, directed electrode holders, to approach the next pose what is known as the holder compensation must be fixed in order to be able to position the electrode holder precisely. This entails difficult, complicated, power-consuming and error-prone mechanisms.